Typically, multiple input, multiple output (MIMO) communications systems make use of space-time coding techniques to enhance the capacity of a communications link and/or to improve its reliability. Furthermore, perfect space-time block codes, such as golden codes for two-transmit antenna MIMO communications systems, were introduced as an energy efficient linear dispersion space-time block code with non-vanishing determinants achieving full-rate and full-diversity while out performing previously known codes.
In general, MIMO communications systems are coherent communications systems that rely on estimating a communications channel at a receiver. Usually, this may be performed by transmitting pilot signals (or sequences) that enable channel estimation. However, the transmission of pilot signals reduces the useful capacity of the communications system. In cases such as high mobility situations where fast fading makes it necessary to frequently re-estimate the communications channel, the reduction in the useful capacity of the communications system by transmitting pilot signals is especially acute.
A technique that may help to reduce the pilot signal overhead problem is to use differential encoding of transmissions. The use of differential encoding eliminates the need for estimating the communications channel at the cost of increased required signal-to-noise ratio (SNR). Another technique that may help to reduce the pilot signal overhead problem is to use blind MIMO, wherein neither the communications channel nor the transmitted codewords are known to the receiver. Blind detection may then be used to recover the transmitted codewords.